We propose to improve the diffraction quality of crystals of thymidylate synthetase we have already obtained, make heavy atom derivatives and obtain an x-ray structure to 4-5A resolution. In addition we plan to explore the crystallization of purified form I of the ternary complex in which the active sites seem to be irreversibly oxidized. These crystals have greater potential for an ultimate high resolution x-ray structure of thymidylate synthetase from L. Casei.